My field assistant, Terry Harlacher, stands next to a dike which has a thickness of
about 15 feet. The waters of the Gulf of Saint Lawrence are in the background.
The dike represents a melt which ascended along a vertical fracture at a time when
the rocks that are now at the surface were at a great depth. The injection of this
melted dike material forced the walls apart and the melt cooled within the fracture to
form an igneous rock which has a different composition than the bedrock. The
bedrock, having been intruded by the dike, is older than the dike. The bedrock
itself is an igneous rock that cooled at great depth; the dike is much younger and
probably cooled at a lesser depth. Subsequently, extended periods of uplift and
erosion have brought both to the surface. Glaciation has removed any soil which
may have formed, and weathering in the few thousand years since the melting of the
glacier has removed more of the less-resistant dike material than the more-resistant
syenite bedrock. Dike thicknesses vary considerably within the area, with some dikes
having a thickness of less than one-inch. Dikes have different compositions and
indicate that different source melts invaded at different times, with younger dikes
cutting both the bedrock and older dikes. By studying the sequence of cross-cutting
relationships among the dikes, the geological history of the region can be inferred.